This invention relates to video signal processing circuitry and, more particularly, to such processing circuitry which eliminates or minimizes the difference between the average levels in adjacent line intervals of a video signal.
In a video signal recorder, such as a video tape recorder (VTR) of the prior art, successive fields are recorded in parallel skewed tracks along a magnetic tape. In order to improve the recording density, the adjacent tracks are recorded without guard bands and, preferably, in partially overlapping relation. Typically, a composite color video signal is recorded by dividing this composite signal into its chrominance and luminance components. The chrominance component then is frequency-converted to a lower frequency band and the luminance component is frequency modulated onto a carrier in a higher frequency band. To eliminate undesired crosstalk between adjacent tracks during a subsequent reproducing operation, a pair of rotary recording heads having different azimuth angles is used. Because of the known principle of azimuth loss, a high frequency signal which is recorded by a magnetic head having one azimuth angle is substantially attenuated when reproduced by another magnetic head having a different azimuth angle. Thus, by recording the processed composite color video signal with heads having different azimuth angles, the higher frequency luminance component which is recorded in the next adjacent track will be substantially attenuated during signal reproduction. However, this principle of azimuth loss, although advantageously used to minimize crosstalk between the higher frequency luminance components, does not apply with respect to the lower frequency chrominance component. That is, azimuth loss provides a useful avoidance of crosstalk only for higher frequency signals. Since the chrominance component is frequency-converted to a lower frequency range, the chrominance component which is recorded in the next adjacent track will be picked up during a reproducing operation so as to introduce undesired crosstalk. To avoid this problem of crosstalk between the chrominance component recorded in adjacent tracks, different carrier frequencies are used for recording the chrominance component in successive tracks. The frequencies of these carriers are selected so as to be in interleaved relationship with each other. Thus, when the chrominance component recorded in a given track is reproduced, the crosstalk component of the chrominance component which is recorded in the next adjacent track also will be reproduced, but this crosstalk component will have a frequency spectrum which is interleaved with the frequency spectrum in the chrominance component reproduced from the given track. Then, a comb filter having nodes at the frequencies corresponding to the interleaved frequencies of the crosstalk chrominance component is used to filter out, or remove, that crosstalk. Thus, the composite color video signal can be recorded and satisfactorily reproduced from magnetic tape with high recording density.
As described in copending U.S. application Ser. No. 770,315, a further increase in recording density generally can be achieved by reducing the width of the parallel tracks. However, azimuth loss is inversely proportional to the width of the tracks. Consequently, if the width of the tracks is reduced, the amount of attenuation in the luminance crosstalk component which is reproduced from an adjacent track likewise is reduced. Hence, interference due to this crosstalk component results in an interference or beat signal having a frequency which differs from that of the luminance or chrominance signals which are reproduced from the scanned track; and this results in a beat or moire pattern on the image which is displayed from the reproduced video signals. Accordingly, it has been proposed in the aforementioned application to record the frequency modulated luminance component with different carriers having an interleaved relation in adjacent tracks. When the luminance component is reproduced and then demodulated, the crosstalk component will appear as an interfering or noise signal which is inverted in phase in successive line intervals. This noise signal of alternating phase will visually cancel in the video picture which is displayed from the reproduced luminance component and, thus, will not be perceived by a viewer.
In the recording operation of the aforementioned application, the carrier frequency upon which the luminance component is frequency modulated varies from one track to the next track by an odd multiple of one-half the horizontal frequency, or (m+1/2)f.sub.H, where m is an integer and f.sub.H is the line frequency, i.e., the frequency of the horizontal synchronizing signal. Accordingly, the frequency of the carrier upon which the luminance component is modulated is shifted from, for example, a lower frequency during the recording of one track to a higher frequency during the recording of the next adjacent track, and then back to the first-mentioned frequency during the recording of the third track, and so on. Since a field interval is recorded in each track, this shift in the carrier frequency is synchronized with the field interval rate. In one embodiment, two different bias voltages are added to the luminance component during alternate field intervals prior to the frequency modulation of the FM carrier by the luminance component. Consequently, a lower bias voltage is added to the luminance component during odd field intervals, that is, during the recording of odd tracks, and a higher bias voltage is added to the luminance component during even fields, that is, during the recording of even tracks. This shift in the bias voltage added to the luminance component results in a corresponding shift in the frequency of the FM carrier.
While the foregoing VTR operates successfully to record and reproduce composite color video signals from a magnetic tape with high recording density, a problem arises due to the fact that, in some instances when an incoming broadcast composite color video signal is received for recording, the average level of the video signal changes from one line interval to the next. That is, in each field (or frame) interval of the broadcasted composite color video signal, the average level of the video signal during even line intervals may be greater or less than the average level of the video signal during odd line intervals. This difference in the average level is present as a difference in the average level of the luminance component. The frequency modulator which is used to frequency modulate the luminance component onto the higher frequency carrier for recording is responsive to the average level of the luminance component. Of course, if this average level remains the same from one line interval to the next, then any change in the carrier frequency attributed to this average level will be the same in each line interval and, thus, the effects thereof will cancel. However, since the average level of the luminance component changes in alternate line intervals, that is, the average level changes from, for example, a relatively lower level during odd line intervals to a relatively higher level during even line intervals, this difference in the average level in succeeding line intervals results in a corresponding change in the frequency of the FM carrier. Thus, the frequencies of the FM carriers which are recorded in adjacent tracks no longer differ from each other by an odd multiple of one-half the horizontal line frequency. That is, the frequency spectra of the frequency modulated luminance components in adjacent tracks no longer exhibit the aforementioned interleaved relation. Consequently, the crosstalk component which appears as an interfering or noise signal in one line is not phase inverted in the successive line. Therefore, these noise signals will not visually cancel; but, rather, will be perceived as an undesired pattern by the viewer when the reproduced video signal is displayed on, for example, a video display device, such as a cathode ray tube.
A typical VTR is provided with a clamp circuit wherein the recorded video signal, and particularly the luminance component thereof, is clamped or referenced either to the tip of the horizontal synchronizing signal or to the pedestal level, depending upon the particular type of clamp circuit which is used. However, the clamp circuit has no effect upon the average level of the video signal. That is, even though such a clamp circuit is provided, the average level of the video signal still may alternate between relatively high and low levels from one line interval to the next. Thus, the clamp circuit is not effective in eliminating undesired interference patterns in the video picture which ultimately is displayed.
A typical VTR also includes an automatic gain control circuit which functions to control the level of the luminance component. However, this gain control circuit generally has a relatively high time constant in order to compensate for variations in the level of the luminance component which are reproduced from adjacent tracks. Hence, changes in the average level of the luminance component from one line interval to the next occur at a rate which is too rapid to be detected and compensated by the gain control circuit. Thus, although level differences from track-to-track, that is, level differences in succeeding field or frame intervals, can be eliminated, differences in the average level from line-to-line cannot.
A change in the average level of a broadcasted video signal from one line to the next will appear as an undesired pattern of stripes in the video picture which is reproduced therefrom. As discussed above, this noise pattern is produced when the broadcasted video signal first is recorded and then is reproduced to provide the displayed video picture. This noise pattern also is formed when the video picture is reproduced directly from the broadcasted video signal. Thus, if the average level of, for example, the luminance component during line intervals n, n+2, . . . differs from the average level of the luminance component during line intervals n+1, n+3, . . . , wherein n is an integer, the quality of the video picture which ultimately is reproduced therefrom is degraded.